// Copyright(C) 1996-2001 Takuya Ooura
// C# port by J.D. Purcell
// You may use, copy, modify this code for any purpose and without fee.

namespace BizHawk.Emulation.Cores.Computers.Commodore64
{
	public class RealFFT
	{
		private int _length;
		private int[] _ip = Array.Empty<int>();
		private double[] _w = Array.Empty<double>();

		public double ForwardScaleFactor { get; private set; }
		public double ReverseScaleFactor { get; private set; }
		public double CorrectionScaleFactor { get; private set; }

		public RealFFT(int length)
		{
			Resize(length);
		}

		public void Resize(int length)
		{
			if (length < 2 || (length & (length - 1)) != 0)
			{
				throw new ArgumentException("FFT length must be at least 2 and a power of 2.", nameof(length));
			}

			ForwardScaleFactor = length;
			ReverseScaleFactor = 0.5d;
			CorrectionScaleFactor = 1.0d / (ForwardScaleFactor * ReverseScaleFactor);
			_length = length;

			var ipLength = 2 + (1 << (Convert.ToInt32(Math.Log(Math.Max(length / 4, 1), 2)) / 2));
			if (_ip.Length < ipLength)
				Array.Resize(ref _ip, ipLength);

			var wLength = length / 2;
			if (_w.Length < wLength)
				Array.Resize(ref _w, wLength);

			_ip.AsSpan().Clear();
			_w.AsSpan().Clear();
		}

		public void ComputeForward(Span<double> buff)
		{
			Compute(buff, false);
		}

		public void ComputeReverse(Span<double> buff)
		{
			Compute(buff, true);
		}

		private void Compute(Span<double> buff, bool reverse)
		{
			if (buff.Length < _length)
			{
				throw new ArgumentException("Buffer length must be greater than or equal to the FFT length.", nameof(buff));
			}

			rdft(_length, reverse, buff, _ip, _w);
		}

		private static void rdft(int n, bool rev, Span<double> a, Span<int> ip, Span<double> w)
		{
			int nw, nc;
			double xi;

			nw = ip[0];
			if (n > (nw << 2))
			{
				nw = n >> 2;
				makewt(nw, ip, w);
			}
			nc = ip[1];
			if (n > (nc << 2))
			{
				nc = n >> 2;
				makect(nc, ip, w, nw);
			}
			if (!rev)
			{
				if (n > 4)
				{
					bitrv2(n, ip, a);
					cftfsub(n, a, w);
					rftfsub(n, a, nc, w, nw);
				}
				else if (n == 4)
				{
					cftfsub(n, a, w);
				}
				xi = a[0] - a[1];
				a[0] += a[1];
				a[1] = xi;
			}
			else
			{
				a[1] = 0.5 * (a[0] - a[1]);
				a[0] -= a[1];
				if (n > 4)
				{
					rftbsub(n, a, nc, w, nw);
					bitrv2(n, ip, a);
					cftbsub(n, a, w);
				}
				else if (n == 4)
				{
					cftfsub(n, a, w);
				}
			}
		}

		/* -------- initializing routines -------- */

		private static void makewt(int nw, Span<int> ip, Span<double> w)
		{
			int j, nwh;
			double delta, x, y;

			ip[0] = nw;
			ip[1] = 1;
			if (nw > 2)
			{
				nwh = nw >> 1;
				delta = Math.Atan(1.0) / nwh;
				w[0] = 1;
				w[1] = 0;
				w[nwh] = Math.Cos(delta * nwh);
				w[nwh + 1] = w[nwh];
				if (nwh > 2)
				{
					for (j = 2; j < nwh; j += 2)
					{
						x = Math.Cos(delta * j);
						y = Math.Sin(delta * j);
						w[j] = x;
						w[j + 1] = y;
						w[nw - j] = y;
						w[nw - j + 1] = x;
					}
					bitrv2(nw, ip, w);
				}
			}
		}

		private static void makect(int nc, Span<int> ip, Span<double> c, int nw)
		{
			int j, nch;
			double delta;

			ip[1] = nc;
			if (nc > 1)
			{
				nch = nc >> 1;
				delta = Math.Atan(1.0) / nch;
				c[nw] = Math.Cos(delta * nch);
				c[nw + nch] = 0.5 * c[nw];
				for (j = 1; j < nch; j++)
				{
					c[nw + j] = 0.5 * Math.Cos(delta * j);
					c[nw + nc - j] = 0.5 * Math.Sin(delta * j);
				}
			}
		}

		/* -------- child routines -------- */

		private static void bitrv2(int n, Span<int> ip, Span<double> a)
		{
			int j, j1, k, k1, l, m, m2;
			double xr, xi, yr, yi;

			ip[2] = 0;
			l = n;
			m = 1;
			while ((m << 3) < l)
			{
				l >>= 1;
				for (j = 0; j < m; j++)
				{
					ip[m + j + 2] = ip[j + 2] + l;
				}
				m <<= 1;
			}
			m2 = 2 * m;
			if ((m << 3) == l)
			{
				for (k = 0; k < m; k++)
				{
					for (j = 0; j < k; j++)
					{
						j1 = 2 * j + ip[k + 2];
						k1 = 2 * k + ip[j + 2];
						xr = a[j1];
						xi = a[j1 + 1];
						yr = a[k1];
						yi = a[k1 + 1];
						a[j1] = yr;
						a[j1 + 1] = yi;
						a[k1] = xr;
						a[k1 + 1] = xi;
						j1 += m2;
						k1 += 2 * m2;
						xr = a[j1];
						xi = a[j1 + 1];
						yr = a[k1];
						yi = a[k1 + 1];
						a[j1] = yr;
						a[j1 + 1] = yi;
						a[k1] = xr;
						a[k1 + 1] = xi;
						j1 += m2;
						k1 -= m2;
						xr = a[j1];
						xi = a[j1 + 1];
						yr = a[k1];
						yi = a[k1 + 1];
						a[j1] = yr;
						a[j1 + 1] = yi;
						a[k1] = xr;
						a[k1 + 1] = xi;
						j1 += m2;
						k1 += 2 * m2;
						xr = a[j1];
						xi = a[j1 + 1];
						yr = a[k1];
						yi = a[k1 + 1];
						a[j1] = yr;
						a[j1 + 1] = yi;
						a[k1] = xr;
						a[k1 + 1] = xi;
					}
					j1 = 2 * k + m2 + ip[k + 2];
					k1 = j1 + m2;
					xr = a[j1];
					xi = a[j1 + 1];
					yr = a[k1];
					yi = a[k1 + 1];
					a[j1] = yr;
					a[j1 + 1] = yi;
					a[k1] = xr;
					a[k1 + 1] = xi;
				}
			}
			else
			{
				for (k = 1; k < m; k++)
				{
					for (j = 0; j < k; j++)
					{
						j1 = 2 * j + ip[k + 2];
						k1 = 2 * k + ip[j + 2];
						xr = a[j1];
						xi = a[j1 + 1];
						yr = a[k1];
						yi = a[k1 + 1];
						a[j1] = yr;
						a[j1 + 1] = yi;
						a[k1] = xr;
						a[k1 + 1] = xi;
						j1 += m2;
						k1 += m2;
						xr = a[j1];
						xi = a[j1 + 1];
						yr = a[k1];
						yi = a[k1 + 1];
						a[j1] = yr;
						a[j1 + 1] = yi;
						a[k1] = xr;
						a[k1 + 1] = xi;
					}
				}
			}
		}

		private static void cftfsub(int n, Span<double> a, Span<double> w)
		{
			int j, j1, j2, j3, l;
			double x0r, x0i, x1r, x1i, x2r, x2i, x3r, x3i;

			l = 2;
			if (n > 8)
			{
				cft1st(n, a, w);
				l = 8;
				while ((l << 2) < n)
				{
					cftmdl(n, l, a, w);
					l <<= 2;
				}
			}
			if ((l << 2) == n)
			{
				for (j = 0; j < l; j += 2)
				{
					j1 = j + l;
					j2 = j1 + l;
					j3 = j2 + l;
					x0r = a[j] + a[j1];
					x0i = a[j + 1] + a[j1 + 1];
					x1r = a[j] - a[j1];
					x1i = a[j + 1] - a[j1 + 1];
					x2r = a[j2] + a[j3];
					x2i = a[j2 + 1] + a[j3 + 1];
					x3r = a[j2] - a[j3];
					x3i = a[j2 + 1] - a[j3 + 1];
					a[j] = x0r + x2r;
					a[j + 1] = x0i + x2i;
					a[j2] = x0r - x2r;
					a[j2 + 1] = x0i - x2i;
					a[j1] = x1r - x3i;
					a[j1 + 1] = x1i + x3r;
					a[j3] = x1r + x3i;
					a[j3 + 1] = x1i - x3r;
				}
			}
			else
			{
				for (j = 0; j < l; j += 2)
				{
					j1 = j + l;
					x0r = a[j] - a[j1];
					x0i = a[j + 1] - a[j1 + 1];
					a[j] += a[j1];
					a[j + 1] += a[j1 + 1];
					a[j1] = x0r;
					a[j1 + 1] = x0i;
				}
			}
		}

		private static void cftbsub(int n, Span<double> a, Span<double> w)
		{
			int j, j1, j2, j3, l;
			double x0r, x0i, x1r, x1i, x2r, x2i, x3r, x3i;

			l = 2;
			if (n > 8)
			{
				cft1st(n, a, w);
				l = 8;
				while ((l << 2) < n)
				{
					cftmdl(n, l, a, w);
					l <<= 2;
				}
			}
			if ((l << 2) == n)
			{
				for (j = 0; j < l; j += 2)
				{
					j1 = j + l;
					j2 = j1 + l;
					j3 = j2 + l;
					x0r = a[j] + a[j1];
					x0i = -a[j + 1] - a[j1 + 1];
					x1r = a[j] - a[j1];
					x1i = -a[j + 1] + a[j1 + 1];
					x2r = a[j2] + a[j3];
					x2i = a[j2 + 1] + a[j3 + 1];
					x3r = a[j2] - a[j3];
					x3i = a[j2 + 1] - a[j3 + 1];
					a[j] = x0r + x2r;
					a[j + 1] = x0i - x2i;
					a[j2] = x0r - x2r;
					a[j2 + 1] = x0i + x2i;
					a[j1] = x1r - x3i;
					a[j1 + 1] = x1i - x3r;
					a[j3] = x1r + x3i;
					a[j3 + 1] = x1i + x3r;
				}
			}
			else
			{
				for (j = 0; j < l; j += 2)
				{
					j1 = j + l;
					x0r = a[j] - a[j1];
					x0i = -a[j + 1] + a[j1 + 1];
					a[j] += a[j1];
					a[j + 1] = -a[j + 1] - a[j1 + 1];
					a[j1] = x0r;
					a[j1 + 1] = x0i;
				}
			}
		}

		private static void cft1st(int n, Span<double> a, Span<double> w)
		{
			int j, k1, k2;
			double wk1r, wk1i, wk2r, wk2i, wk3r, wk3i;
			double x0r, x0i, x1r, x1i, x2r, x2i, x3r, x3i;

			x0r = a[0] + a[2];
			x0i = a[1] + a[3];
			x1r = a[0] - a[2];
			x1i = a[1] - a[3];
			x2r = a[4] + a[6];
			x2i = a[5] + a[7];
			x3r = a[4] - a[6];
			x3i = a[5] - a[7];
			a[0] = x0r + x2r;
			a[1] = x0i + x2i;
			a[4] = x0r - x2r;
			a[5] = x0i - x2i;
			a[2] = x1r - x3i;
			a[3] = x1i + x3r;
			a[6] = x1r + x3i;
			a[7] = x1i - x3r;
			wk1r = w[2];
			x0r = a[8] + a[10];
			x0i = a[9] + a[11];
			x1r = a[8] - a[10];
			x1i = a[9] - a[11];
			x2r = a[12] + a[14];
			x2i = a[13] + a[15];
			x3r = a[12] - a[14];
			x3i = a[13] - a[15];
			a[8] = x0r + x2r;
			a[9] = x0i + x2i;
			a[12] = x2i - x0i;
			a[13] = x0r - x2r;
			x0r = x1r - x3i;
			x0i = x1i + x3r;
			a[10] = wk1r * (x0r - x0i);
			a[11] = wk1r * (x0r + x0i);
			x0r = x3i + x1r;
			x0i = x3r - x1i;
			a[14] = wk1r * (x0i - x0r);
			a[15] = wk1r * (x0i + x0r);
			k1 = 0;
			for (j = 16; j < n; j += 16)
			{
				k1 += 2;
				k2 = 2 * k1;
				wk2r = w[k1];
				wk2i = w[k1 + 1];
				wk1r = w[k2];
				wk1i = w[k2 + 1];
				wk3r = wk1r - 2 * wk2i * wk1i;
				wk3i = 2 * wk2i * wk1r - wk1i;
				x0r = a[j] + a[j + 2];
				x0i = a[j + 1] + a[j + 3];
				x1r = a[j] - a[j + 2];
				x1i = a[j + 1] - a[j + 3];
				x2r = a[j + 4] + a[j + 6];
				x2i = a[j + 5] + a[j + 7];
				x3r = a[j + 4] - a[j + 6];
				x3i = a[j + 5] - a[j + 7];
				a[j] = x0r + x2r;
				a[j + 1] = x0i + x2i;
				x0r -= x2r;
				x0i -= x2i;
				a[j + 4] = wk2r * x0r - wk2i * x0i;
				a[j + 5] = wk2r * x0i + wk2i * x0r;
				x0r = x1r - x3i;
				x0i = x1i + x3r;
				a[j + 2] = wk1r * x0r - wk1i * x0i;
				a[j + 3] = wk1r * x0i + wk1i * x0r;
				x0r = x1r + x3i;
				x0i = x1i - x3r;
				a[j + 6] = wk3r * x0r - wk3i * x0i;
				a[j + 7] = wk3r * x0i + wk3i * x0r;
				wk1r = w[k2 + 2];
				wk1i = w[k2 + 3];
				wk3r = wk1r - 2 * wk2r * wk1i;
				wk3i = 2 * wk2r * wk1r - wk1i;
				x0r = a[j + 8] + a[j + 10];
				x0i = a[j + 9] + a[j + 11];
				x1r = a[j + 8] - a[j + 10];
				x1i = a[j + 9] - a[j + 11];
				x2r = a[j + 12] + a[j + 14];
				x2i = a[j + 13] + a[j + 15];
				x3r = a[j + 12] - a[j + 14];
				x3i = a[j + 13] - a[j + 15];
				a[j + 8] = x0r + x2r;
				a[j + 9] = x0i + x2i;
				x0r -= x2r;
				x0i -= x2i;
				a[j + 12] = -wk2i * x0r - wk2r * x0i;
				a[j + 13] = -wk2i * x0i + wk2r * x0r;
				x0r = x1r - x3i;
				x0i = x1i + x3r;
				a[j + 10] = wk1r * x0r - wk1i * x0i;
				a[j + 11] = wk1r * x0i + wk1i * x0r;
				x0r = x1r + x3i;
				x0i = x1i - x3r;
				a[j + 14] = wk3r * x0r - wk3i * x0i;
				a[j + 15] = wk3r * x0i + wk3i * x0r;
			}
		}

		private static void cftmdl(int n, int l, Span<double> a, Span<double> w)
		{
			int j, j1, j2, j3, k, k1, k2, m, m2;
			double wk1r, wk1i, wk2r, wk2i, wk3r, wk3i;
			double x0r, x0i, x1r, x1i, x2r, x2i, x3r, x3i;

			m = l << 2;
			for (j = 0; j < l; j += 2)
			{
				j1 = j + l;
				j2 = j1 + l;
				j3 = j2 + l;
				x0r = a[j] + a[j1];
				x0i = a[j + 1] + a[j1 + 1];
				x1r = a[j] - a[j1];
				x1i = a[j + 1] - a[j1 + 1];
				x2r = a[j2] + a[j3];
				x2i = a[j2 + 1] + a[j3 + 1];
				x3r = a[j2] - a[j3];
				x3i = a[j2 + 1] - a[j3 + 1];
				a[j] = x0r + x2r;
				a[j + 1] = x0i + x2i;
				a[j2] = x0r - x2r;
				a[j2 + 1] = x0i - x2i;
				a[j1] = x1r - x3i;
				a[j1 + 1] = x1i + x3r;
				a[j3] = x1r + x3i;
				a[j3 + 1] = x1i - x3r;
			}
			wk1r = w[2];
			for (j = m; j < l + m; j += 2)
			{
				j1 = j + l;
				j2 = j1 + l;
				j3 = j2 + l;
				x0r = a[j] + a[j1];
				x0i = a[j + 1] + a[j1 + 1];
				x1r = a[j] - a[j1];
				x1i = a[j + 1] - a[j1 + 1];
				x2r = a[j2] + a[j3];
				x2i = a[j2 + 1] + a[j3 + 1];
				x3r = a[j2] - a[j3];
				x3i = a[j2 + 1] - a[j3 + 1];
				a[j] = x0r + x2r;
				a[j + 1] = x0i + x2i;
				a[j2] = x2i - x0i;
				a[j2 + 1] = x0r - x2r;
				x0r = x1r - x3i;
				x0i = x1i + x3r;
				a[j1] = wk1r * (x0r - x0i);
				a[j1 + 1] = wk1r * (x0r + x0i);
				x0r = x3i + x1r;
				x0i = x3r - x1i;
				a[j3] = wk1r * (x0i - x0r);
				a[j3 + 1] = wk1r * (x0i + x0r);
			}
			k1 = 0;
			m2 = 2 * m;
			for (k = m2; k < n; k += m2)
			{
				k1 += 2;
				k2 = 2 * k1;
				wk2r = w[k1];
				wk2i = w[k1 + 1];
				wk1r = w[k2];
				wk1i = w[k2 + 1];
				wk3r = wk1r - 2 * wk2i * wk1i;
				wk3i = 2 * wk2i * wk1r - wk1i;
				for (j = k; j < l + k; j += 2)
				{
					j1 = j + l;
					j2 = j1 + l;
					j3 = j2 + l;
					x0r = a[j] + a[j1];
					x0i = a[j + 1] + a[j1 + 1];
					x1r = a[j] - a[j1];
					x1i = a[j + 1] - a[j1 + 1];
					x2r = a[j2] + a[j3];
					x2i = a[j2 + 1] + a[j3 + 1];
					x3r = a[j2] - a[j3];
					x3i = a[j2 + 1] - a[j3 + 1];
					a[j] = x0r + x2r;
					a[j + 1] = x0i + x2i;
					x0r -= x2r;
					x0i -= x2i;
					a[j2] = wk2r * x0r - wk2i * x0i;
					a[j2 + 1] = wk2r * x0i + wk2i * x0r;
					x0r = x1r - x3i;
					x0i = x1i + x3r;
					a[j1] = wk1r * x0r - wk1i * x0i;
					a[j1 + 1] = wk1r * x0i + wk1i * x0r;
					x0r = x1r + x3i;
					x0i = x1i - x3r;
					a[j3] = wk3r * x0r - wk3i * x0i;
					a[j3 + 1] = wk3r * x0i + wk3i * x0r;
				}
				wk1r = w[k2 + 2];
				wk1i = w[k2 + 3];
				wk3r = wk1r - 2 * wk2r * wk1i;
				wk3i = 2 * wk2r * wk1r - wk1i;
				for (j = k + m; j < l + (k + m); j += 2)
				{
					j1 = j + l;
					j2 = j1 + l;
					j3 = j2 + l;
					x0r = a[j] + a[j1];
					x0i = a[j + 1] + a[j1 + 1];
					x1r = a[j] - a[j1];
					x1i = a[j + 1] - a[j1 + 1];
					x2r = a[j2] + a[j3];
					x2i = a[j2 + 1] + a[j3 + 1];
					x3r = a[j2] - a[j3];
					x3i = a[j2 + 1] - a[j3 + 1];
					a[j] = x0r + x2r;
					a[j + 1] = x0i + x2i;
					x0r -= x2r;
					x0i -= x2i;
					a[j2] = -wk2i * x0r - wk2r * x0i;
					a[j2 + 1] = -wk2i * x0i + wk2r * x0r;
					x0r = x1r - x3i;
					x0i = x1i + x3r;
					a[j1] = wk1r * x0r - wk1i * x0i;
					a[j1 + 1] = wk1r * x0i + wk1i * x0r;
					x0r = x1r + x3i;
					x0i = x1i - x3r;
					a[j3] = wk3r * x0r - wk3i * x0i;
					a[j3 + 1] = wk3r * x0i + wk3i * x0r;
				}
			}
		}

		private static void rftfsub(int n, Span<double> a, int nc, Span<double> c, int nw)
		{
			int j, k, kk, ks, m;
			double wkr, wki, xr, xi, yr, yi;

			m = n >> 1;
			ks = 2 * nc / m;
			kk = 0;
			for (j = 2; j < m; j += 2)
			{
				k = n - j;
				kk += ks;
				wkr = 0.5 - c[nw + nc - kk];
				wki = c[nw + kk];
				xr = a[j] - a[k];
				xi = a[j + 1] + a[k + 1];
				yr = wkr * xr - wki * xi;
				yi = wkr * xi + wki * xr;
				a[j] -= yr;
				a[j + 1] -= yi;
				a[k] += yr;
				a[k + 1] -= yi;
			}
		}

		private static void rftbsub(int n, Span<double> a, int nc, Span<double> c, int nw)
		{
			int j, k, kk, ks, m;
			double wkr, wki, xr, xi, yr, yi;

			a[1] = -a[1];
			m = n >> 1;
			ks = 2 * nc / m;
			kk = 0;
			for (j = 2; j < m; j += 2)
			{
				k = n - j;
				kk += ks;
				wkr = 0.5 - c[nw + nc - kk];
				wki = c[nw + kk];
				xr = a[j] - a[k];
				xi = a[j + 1] + a[k + 1];
				yr = wkr * xr + wki * xi;
				yi = wkr * xi - wki * xr;
				a[j] -= yr;
				a[j + 1] = yi - a[j + 1];
				a[k] += yr;
				a[k + 1] = yi - a[k + 1];
			}
			a[m + 1] = -a[m + 1];
		}
	}
}
